Wormgears are used when large gear reductions are needed. It is common for wormgears to have reductions of 20:1, and even up to 300:1 or greater. A worm is used to reduce speed and increase torque. For each complete turn of the worm shaft the gear shaft advances only one tooth of the gear. Unlike ordinary gears, the motion is not reversible, a worm can drive a gear to reduce speed but, in general, a gear cannot drive a worm to increase it. As the speed is reduced the torque to the drive increases correspondingly. Worm gears are a compact, efficient means of substantially decreasing speed and increasing torque. Wormgears operate under high sliding velocity and moderate loads. The sliding action produces friction that produces higher operating temperatures than those that occur in other gear drives. Lubricants for wormgears are often formulated with base oils that have high viscosity indexes and low traction coefficients to resist the thinning due to high temperatures and reduce frictional energy loss.
It has been shown by L. A. Bronshteyn and J. H. Kreiner, “Energy Efficiency of Industrial Oils,” STLE paper 99-AM-2, (1999), that the property most relevant to wormgear energy efficiency is the traction coefficient of the finished lubricant used in the wormgear. The percent energy efficiency of any wormgear drive [i.e. where the worm drives the wheel] has been shown to be a function of the system's friction coefficient and worm geometry according to the following relationship [Dudley's Gear Handbook, 2nd Edition, D. P. Townsend, ed., McGraw Hill, Inc., (1992), p12.11]:Energy Efficiency=100*[cos φ−f tan λ]/[cos φ+tan λ]                where φ=normal pressure angle                    λ=pinion lead angle            f=coefficient of friction.                        
Previous studies by W. R. Murphy et. al., “The Effect of Lubricant Traction on Wormgear Efficiency,” AGMA P254.33 (1981), have shown that the overall frictional characteristics for many commercial wormgear systems are virtually identical to the traction coefficients of the lubricants used in those systems. Therefore, it is possible to estimate the potential energy efficiencies of different lubricant formulations by substituting the traction coefficient for the friction coefficient in the above equation. This provides a good measure of energy efficiency since other lubricant properties such as viscosity, viscosity modifier type and content, and friction modifier content have been shown to exert only a relatively minor influence on the energy efficiency of commercial wormgear drives.
Wormgears by their very nature, transfer power to their mating wormwheels under essentially pure sliding conditions. The frictional heat that is generated by this movement manifests itself as a loss of power transmission efficiency [horsepower loss] and leads to a rise in temperature of oil in the oil reservoir, or bulk oil. In non-thermostatted units, the greater the efficiency transfer characteristics of the lubricant, the lower the equilibrium oil temperature observed. As a result, the potential energy savings of any lubricant in a wormgear drive application should be directly related to its power efficiency at the equilibrium oil temperature of the oil in the reservoir.
In the past, finished lubricant formulators have selected base oils and additives to improve the traction coefficient of the finished lubricant made from them. Examples of additives, referred to as friction modifiers, used to improve the traction coefficients of finished fractions have been taught in EP0973854, U.S. Pat. No. 4,634,543, and WO9524458. Polyalphaolefins (PAOs) are known to have lower friction and hence increased mechanical efficiency and reduced frictional heat loss across the full spectrum of mechanical loads from wormgears to traction drives, and do so over a wider range of operating conditions than mineral oil lubricants. Certain preferred polyalphaolefins with low friction coefficients are taught in U.S. Pat. No. 4,827,064. Often 3 to 10 percent fatty or synthetic fatty oils are added to wormgear oils to reduce friction.
A presentation by C. L. Bullock, “Innovations in Synthetic Lubricant Basestocks,’ ICIS-LOR World Base Oil Conference (2004) demonstrated that blends of lower-traction coefficient PAOs (SpectraSyn Ultra™) with Group I and Group II formulations provided 20–30% wear reduction in heavy duty engines. Similar benefits that might be achieved using lower-traction coefficient Fischer-Tropsch distillate fractions in place of PAOs were not taught or anticipated by this presentation. An ExxonMobil Chemical presentation titled “SpectraSyn Ultra™—The Ultra Performance PAO,” copyright 2003, demonstrated improved wormgear efficiency with lower-traction coefficient PAOs with kinematic viscosities between about 150 and 1,000 cSt at 100 degrees C. The SpectraSyn Ultra™ base oils also had high film thicknesses.
U.S. patent application Ser. No. 10/301,391 teaches blending of Fischer-Tropsch derived distillate fractions with kinematic viscosities of about 2 but less than 3 cSt at 100 degrees C. with petroleum derived base oils to produce a base oil blend with desirable low volatility and a viscosity of about 3 or greater. Benefits relating to reduced traction coefficients were not taught. It did not teach that wormgear lubricants with kinematic viscosities between ISO 68 and ISO 680 could be blended with the blended base oils of the invention.
U.S. patent application Ser. No. 10/704,031 teaches blending of an isomerized Fischer-Tropsch derived bottoms fraction with either a conventional petroleum-derived base oil or a Fischer-Tropsch derived base oil to reduce the pour point of the blend. This application does not teach that there is any benefit for friction reduction.
What we have discovered are methods of operating wormgear drives at higher energy efficiency than what was previously possible with finished lubricants made from petroleum derived lubricating base oils and/or conventional polyalphaolefins. What we also have found is a process to reduce the traction coefficients of higher-traction coefficient lubricating base oils to make them more suitable for use in a wormgear drive by blending them with isomerized Fischer-Tropsch distillate fractions having very low traction coefficients. In addition, we have discovered a new composition of wormgear lubricant that provides superior energy efficiency.